Just as electrical power lines may transmit transient surges or other voltage irregularities to electrical equipment intended to be powered therefrom, so equipment in line of such power transmission may cause interfering electrical signals or noise to be conducted back to an intervening electrical bus and even to the power lines.
Such conducted emissions (CE) may degrade computer performance, television picture quality, and the functioning of other electrical equipment powered from such lines. CE types of interference may be generated by computers, radio and TV receivers, motors, switching apparatus, or almost any electrical equipment having (or causing) a varying or random emission, whether as intended output or otherwise.
Filters are useful in reducing such interference, and it is conventional--in determining the desirability and effectiveness of filters--to measure conducted emissions with an instrument often called an EMI meter or spectrum analyzer. Such a meter has a logarithmic (dB) scale and is responsive via a voltage probe or current loop throughout a frequency range up to 30 MHz or higher.
Plotting the results of such measurements reveals whatever interference peaks and bands may exist, so that adequate filters can be designed and be connected to (or be installed in) the equipment under test (EUT) to limit such interference to a tolerable level at all frequencies. Designing such filters may fairly be viewed now as principally an art rather than a science.
Examples of persons who are both workers and writers in this field include Lon M. Schneider and Alphonse A. Toppetto, as well as the present inventor. Their published articles are of more interest than are most EMI patents, although Toppetto U.S. Pat. No. 4,263,549 discloses apparatus for determining "differential mode" and "common" mode noise. Common mode (CM) current flows from phase and neutral lines to ground, whereas "differential mode" (DMI) current flows from a phase line to a neutral line. A need exists for methods and means of empirically determining with greater facility what filters are most suitable to combat both such types of conducted emissions.